Study of Dye-Sensitized Solar Cells by Scanning Electron Micrograph Observation and Thickness Optimization of Porous TiO2 Electrodes

1 Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan 2 Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland 3 New Business Research Center, JGC Catalysts and Chemicals Ltd., 13-2 Kitaminato-Machi, Wakamatsu-Ku, Kitakyuushu, Fukuoka 808-0027, Japan


Introduction
Dye-sensitized solar cells (DSCs) have recently been developed as a cost-effective photovoltaic system due to their lowcost materials and facile processing [1][2][3][4].The production of DSCs involves chemical and thermal processes but no vacuum is involved.Therefore, DSCs can be fabricated without using expensive equipment.The use of dyes and nanocrystalline TiO 2 is one of the most promising approaches to realize both high performance and low cost.A high light-toelectricity conversion efficiency can be obtained from a large surface area of porous TiO 2 electrodes on which the dyes can be sufficiently adsorbed.
In order to fabricate DSC devices characterized by high efficiency (>10%) and/or long-term stability, further understanding and state-of-art techniques are necessary.The key points of the DSC mechanism are as follows (1) Ultrafast electron injection from the dye to the TiO 2 electrode (<20 femtoseconds) [5].(2) Fast dye regeneration via reduction by I − (100 nanoseconds without voltage bias) [6].(3) The very long electron lifetime in the TiO 2 electrode to I − 3 (>0.1 second below 0.7 V forward bias) [7].(4) The large roughness factor of the TiO 2 electrode (>1600) [8].(5) The diode effect of Ru dye on the TiO 2 electrode [8].(6) Photon adsorption controlled by the thickness of the TiO 2 layers (transparent layer, d = 20 nm, 12-14 μm thickness; scattering layer, d = 400 nm, 4-5 μm thickness) [9].Each point listed above is necessary for high-efficiency DSCs.One of the key materials is the TiO 2 electrode, which has been fabricated by using sol-gel screen printing.With this method, the TiO 2 particle size and the thickness of the nanocrystalline-TiO 2 layer ("X" in Figure 1) can be varied, allowing for control of the photovoltage and photocurrent.Thus, we can optimize the design of the porous TiO 2 electrode, the sensitizing dye and the electrolyte.
In this paper, we study the surface and cross-sectional structure of double-layered porous TiO 2 electrodes by SEM observation.We found significant differences between the SEM images of the original structure composed of submicron particles and those of a structure composed of a mixture of submicron particles and nanoparticles.The two types of cross-sectional samples were prepared either by mechanically breaking the substrate or using an Ar-ion beam etching technique, resulting in confirmation of the significant difference in the appearance of the two samples and the observation of the interesting angular and horned structure of the 400 nm diameter particles in the cross-sectional view.Subsequently, the effect of the TiO 2 particle size (d = 20 nm, 42 nm and 400 nm) and the thickness of the nanocrystalline-TiO 2 electrode on the photovoltaic properties of DSCs are evaluated with four kinds of sensitizing dyes (D149, K19, N719 and Z907) and two kinds of electrolytes (acetonitrile (AcCN) based and ionic-liquid (IL) electrolytes) in order to achieve high conversion efficiency.

Porous TiO 2
Electrodes.To prepare the working electrodes, the fluorine-doped tin oxide (FTO) glass that was used as the current collector (Nippon Sheet Glass, Solar, 4 mm thickness) was first cleaned in a detergent solution using an ultrasonic bath for 15 minutes and then rinsed with water and ethanol.After treatment in an UV-O 3 system for 18 minutes, the FTO glass plates were immersed in a 40 mM aqueous solution of TiCl 4 at 70 • C for 30 minutes and washed with water and ethanol.A layer of nanocrystalline-TiO 2 paste was coated onto the FTO glass plates by screen printing and kept in a clean box for 3 minutes so that the paste could settle to reduce the irregularity of the surface and then dried for 6 minutes at 125 • C.This screen-printing procedure with the paste (coating, storing and drying) was repeated to obtain a working electrode of appropriate thickness [9].By adjusting the thickness of the TiO 2 pastes, the TiO 2 electrodes were designed as shown in Figure 1.The porous TiO 2 layers of 20 nm [9], 42 nm [11] and 400 nm [9] were transparent, semitransparent and scattering, respectively.For the scattering layer, two coatings of macrocrystalline-TiO 2 paste were deposited by screen printing, resulting in a lightscattering TiO 2 layer of 4-5 μm thickness containing 400 nmsized anatase particles.The electrodes coated with the TiO 2 pastes were gradually heated under an air flow at 325 • C for 5 minutes, at 375 • C for 5 minutes, at 450 • C for 15 minutes and 500 • C for 15 minutes.
The sintered TiO 2 film was treated with 40 mM TiCl 4 solution as described above, rinsed with water and ethanol and sintered again at 500 • C for 30 minutes.After cooling to 80 • C, the TiO 2 electrode was immersed in a 0.5 mM dye solution in a mixture of acetonitrile and tert-butyl alcohol (v/v: 1 : 1) and kept at room temperature for 20-24 hours (Ru dyes) or 4 hours (D149 with 1.0 mM chenodeoxy cholic acid) to complete the sensitizer uptake.

Pt Counter Electrodes.
To prepare the counter electrode, a hole was drilled in the FTO glass (LOF Industries, TEC 15 Ω/ , 2.2 mm thickness) by sand blasting.The perforated glass sheet was washed with H 2 O as well as with a 0.1 M HCl solution in ethanol and cleaned by ultrasound in an acetone bath for 10 minutes.After removing residual organic contaminants by heating in air for 15 minutes at 400 • C, the Pt catalyst was deposited on the FTO glass by coating with a drop of H 2 PtCl 6 solution (3 mg Pt in 1 mL ethanol) and repeating the heat treatment at 400 • C for 15 minutes.
The cell was placed in a small vacuum chamber for a few seconds to remove the inside air.Exposing it again to ambient pressure caused the electrolyte to be driven into the cell.Finally, the hole was sealed using a hot-melt liner low density polyethylene film (Bynel 4164, 35 μm thickness, Du-Pont) and a cover glass (0.1 mm thickness).In order to have good electrical contact between the connections and the photovoltaic measurement equipment, the edge of the FTO outside of the cell was scraped slightly with sandpaper or a file.A solder (Cerasolza, Asahi Glass) was applied on each side of the FTO electrodes.
2.5.Measurements.Photovoltaic measurements employed an AM 1.5 solar simulator (100 mW cm −2 ).Masks made of black plastic tape were attached on the Arctop filter to reduce the scattering of light [17].The intensity of the simulated light was calibrated by using a reference Si photodiode equipped with an IR cutoff filter (KG-3, Schott) in order to reduce the mismatch in the region of 350-750 nm between the simulated light and AM 1.5 to less than 2% [18].I-V curves were obtained by applying an external bias voltage to the cell and measuring the generated photocurrent with a digital source meter (Keithley 2400, Keithley Instruments).

Results and Discussions
3.1.SEM Observation of Nanocrystalline-TiO 2 Porous Electrodes. Figure 2(a) shows the SEM images of submicron TiO 2 particles that were used in the light-scattering layer (Figure 1(b)).The particles were a pyramid-like angular shape.The light-scattering TiO 2 layer was a mixture of nanosized and submicron TiO 2 particles [9].In the SEM observations, the angular shape of the submicron TiO 2 particles could not be seen in the light-scattering layer (Figure 2(b)).The submicron TiO 2 particles were surrounded by nanoparticles.The outline of the angular shape can be seen through the surrounding nanoparticles.
Figure 3 shows cross-sectional SEM images of a porous TiO 2 electrode that was prepared by breaking the substrate with a glass cutter.It is noticeable that the electrode has a double-layered structure, as illustrated in Figure 1(b).The magnified view of the light-scattering layer (Figure 3(b)) is similar to Figure 2(b); the submicron particles were coated by nanoparticles in the cross-sectional view of a sample prepared by mechanically breaking the substrate.
Figure 4 shows the cross-sectional images of a nanocrystalline-TiO 2 porous electrode prepared by polishing with an Ar-ion beam (the preparation and observation of the samples were done by KOBELCO Research Institute, Inc., Japan).The observed nanocrystalline-TiO 2 porous electrode has a double-layered structure on a glass substrate similar to Figure 1(b).The nanocrystalline-TiO 2 layer is smooth and flat, while the submicroncrystalline-TiO 2 layer is rough due to the large particle size (Figure 4(a)).The nanocrystalline-TiO 2 layer appears very homogeneous at 5 000-times magnification (Figure 4(b)).At 50 000 times magnification (Figure 4(c)), small cracks can be seen in the layer due to shrinkage caused by the heating procedure.At 200 000 times magnification, the nanoparticles can be observed (Figure 4(d)), but the image is not clear because no metal was deposited by spattering in order to observe them.Figure 4(e) shows the cross-section of the light-scattering layer by submicroncrystalline-TiO 2 particles.The material of this layer is a mixture of 90% of submicroncrystalline-TiO 2 particles and 10% of nanocrystalline-TiO 2 particles [9].We can see the exact cross-sectional view of the submicroncrystalline-TiO 2 particles (Figure 4(f)).The size of the cross-sectioned submicron crystals is between 100 nm and 500 nm.This size distribution is due to the Ar-ion beam etching, which cut the crystal at the center and/or the edge.It is surprising that the structure of the submicroncrystalline-TiO 2 particles is rather complicated (angular and horned) in the cross-sectional view, which cannot be observed in the SEM images of Figure 2(a).

I-V Curves with
Variation of TiO 2 Thickness.Figure 5 shows the I-V curves of the DSCs with double-layer TiO 2 (d = 20 nm and 400 nm) electrodes (Figure 1(b)).
The thickness of the nanocrystalline-TiO 2 layer was adjusted between 2.8 μm to 17.4 μm.The sensitizing dye was a Ru dye (N719), and the electrolyte was an AcCN-based electrolyte.It was observed that the thicker layer gave higher photocurrent and lower photovoltage.On the other hand, the thinner layer gave lower photocurrent and higher photovoltage.This is a typical phenomenon in DSCs regardless of the kind of particles and dyes.In the next sections, we discuss the photovoltaic characteristics for two sizes of TiO 2 nanoparticles (d = 42 nm and 20 nm).

Single-Layer Electrodes.
Figure 6 shows the photovoltaic characteristics of single-layer electrodes with a semitransparent layer of 42 nm TiO 2 particles (Figure 1(a)).The combinations of sensitizing dyes and electrolytes were Ru dye (N719)/AnCN-based electrolyte and Ru dye (Z907)/IL electrolyte .
For both combinations of dye and electrolyte, the open-circuit photovoltage (Voc) decreased linearly with increasing thickness of the nanocrystalline-TiO 2 electrode (Figure 6(a)).
However, the behavior of the short-circuit photocurrent density (Jsc) (Figure 6(b)) was different for the electrolytes.For the AcCN-based electrolyte, the Jsc increased and became saturated with increasing thickness.On the other hand, theJsc for the IL electrolyte reached a maximum around 14 μm.At the thin TiO 2 electrode (4 μm thickness), the values of Jsc for the AcCN-based (11.8 mA cm −2 ) and IL (10.5 mA cm −2 ) electrolytes were similar; the difference was only 12% (1.3 mA cm −2 ).Upon increasing the thickness, the difference increased to 42% for the 21 μm thick films.This arises from the high viscosity and slow I − /I − 3 diffusion in the IL electrolyte [16].The redox couple (I − /I − 3 ) can diffuse readily across the thin nanocrystalline-TiO 2 layer and sustain the low photocurrent.However, transport is impaired in thicker nanocrystalline electrode films where larger photocurrents must be sustained.By contrast, the viscosity of AcCN is so low that high photocurrent can be supported even for thick nanocrystalline-TiO 2 layers.
The fill factor (FF) was not affected by variations in the thickness of the nanocrystalline-TiO 2 layer (Figure 6(c)).
Consequently, the conversion efficiency reached a maximum (Figure 6(d)).Nevertheless, the viscosity and the diffusion coefficient of I − /I − 3 differed by a factor of 100 [16], the AcCN-based and IL electrolytes gave peaks in the same position for the 14-16 μm thickness.

Double-Layer Electrodes.
Figure 7 shows the photovoltaic characteristics of the double-layer electrodes (Figure 1(b)).The particle sizes for the transparent and lightscattering layers were 20 nm and 400 nm, respectively.In this paper, the thickness of the transparent layer (d = 20 nm; "X" in Figure 1(b)) was varied.Four combinations of sensitizing dyes and electrolytes were used: Ru dye (N719)/AnCNbased electrolyte , Ru dye (K19)/IL electrolyte , organic dye (D149)/AnCN-based electrolyte and organic dye (D149)/IL electrolyte .
In all cases, Voc decreased linearly with increasing the thickness of the nanocrystalline-TiO 2 electrode (Figure 7(a)).This phenomenon was the same as that of the single-layer system (Figure 6(a)).It was remarkable that the highest Voc was over 900 mV for the nanocrystalline-TiO 2 electrode (3 μm) using Ru dye (N719) and AcCN-based electrolyte.with increasing thickness.On the other hand, using the IL electrolyte, the Jsc had a peak at a thickness of around 6 μm.The optimal thickness for the 20 nm particles (Figure 7 3 ) could diffuse more effectively than into the smaller pores created by the smaller particles (d = 20 nm).Therefore, the larger particles gave a Jsc peak at a larger thickness than the smaller particles.
The FF was not greatly affected by variations in the thickness of the nanocrystalline-TiO 2 layer (Figure 7(c)).In the case of the organic dye (D149) with IL electrolyte, however, the FF decreased as a result of the high photocurrent and the low diffusion coefficient of the redox couple.
Consequently, the conversion efficiency reached a maximum (Figure 7(d)).The AcCN-based and IL electrolytes gave peaks at 14 μm and 6 μm thickness, respectively.The important point is that the position of the peaks was unrelated to the nature of the Ru dye and the organic dye, in spite of the 4-fold difference between their absorption coefficients (N719: 13,900 M −1 cm −1 , K19: 18,200 M −1 cm −1 , D149: 68,700 M −1 cm −1 ).At first, we expected that the organic dye would yield the highest efficiency when the thickness of the nanocrystalline-TiO 2 layer was small; however, the peak of efficiency was determined by the electrolyte and the particle size, not by the type of sensitizer.

Conclusions
The structure of the cross-section of the double-layered TiO 2 electrode, which was prepared by Ar-ion beam etching, has been observed by SEM for the first time.We confirmed the difference between the cross-sectional structures prepared by mechanically breaking the substrate and Ar-ion beam International Journal of Photoenergy etching and observed the unique structure of the 400 nm TiO 2 particles with respect to the light-scattering effect.
In order to optimize the design of DSCs for maximal conversion efficiency, the TiO 2 particle size and the thickness of the nanocrystalline-TiO 2 layer were varied and tested with several sensitizing dyes and electrolytes.For the singlelayer electrodes (semitransparent, d = 42 nm), the maximal efficiencies were 6.6% at 14 μm and 9.9% at 16 μm with Z907/IL and N719/AcCN , respectively.For the doublelayer electrodes (transparent layer: d = 20 nm; lightscattering layer: d = 400 nm), the maximal efficiencies were 7.0% at 6 μm, 6.3% at 6 μm, 10.2% at 14 μm and 9.0% at 14 μm with K19/IL , D149/IL , N719/AcCN and D149/AcCN , respectively.The optimal thickness of the nanocrystalline-TiO 2 layer depends on the electrolyte and TiO 2 particle size but not on the absorption coefficient of the dye.
From these experimental results, we can estimate the conversion efficiency of a DSC for an ideal case where Voc, Jsc and FF attain their maximal values.The best values were a Voc of 900 mV (Figure 7  a high absorption coefficient that displays a strong blocking effect against charge recombination.We hope that this report is helpful for the development of DSCs for practical applications.

Figure 1 :
Figure 1: Structure of DSC with single-layer (a) and double-layer (b) structures of nanocrystalline-TiO 2 electrodes.X's in figures show the thickness of nanocrystalline-TiO 2 electrodes that was varied for the optimization of high-efficiency DSC (Figures 5, 6 and 7).

Figure 3 :
Figure 3: Cross-sectional views of a double-layer electrode prepared by mechanically breaking the substrate.The white dotted quadrangle in Figure 3(a) indicates the zoomed-in area shown in Figure 3(b).

Figure 7 (Figure 4 :
Figure 4: Cross-sectional SEM images of a porous double-layered TiO 2 electrode polished with an Ar-ion beam (nanocrystalline-TiO 2 layer: 14 μm thick; submicron crystalline-TiO 2 layer: 5 μm thick).(a) is the most zoomed-out image.White dotted quadrangles show the zoomed-in area for the other images.

Figure 5 :
Figure 5: Photo I-V curves in relation to the thickness of doublelayer nanocrystalline-TiO 2 electrodes (as shown in Figure 1(b)) with Ru dye (N719) and AcCN-based electrolyte.
(b)) was half that obtained for the 42 nm particles (Figure 6(b)).The larger particles (d = 42 nm, Figure 6(b)) produced larger pores into which the redox couple (I − /I −